A process for the selective deposition of metal on substrates is disclosed. The process is used to form wires or conducting lines on substrates in processes for fabricating printed circuits.
Printed circuits are typically fabricated by plating conducting metal wires on substrates made of non-conducting material such as a ceramic substrates or on resistive metal films. The metal wires are formed on the substrate in a particular pattern. Typically, the pattern is either a pre-formed stencil mask, or is formed directly on the substrate in a layer of energy sensitive material using lithographic techniques. The pattern is used to form the metal on the substrate, either by transferring the pattern into a metal layer underlying the resist material or by depositing metal in the interstices of a pattern formed on the substrate.
The metal is formed on the substrate using techniques such as electroplating and electroless plating. In electroless metal plating, the substrate is immersed in an aqueous metal salt solution to plate metal on the substrate.
If electroless plating is used in a process for fabricating printed circuits, the metal formed on the substrate must adequately adhere to the substrate. Metal does not adequately adhere to the substrate if it becomes easily dislodged from the substrate either after being plated on the substrate or during subsequent processing of the printed circuit.
Adequate adhesion is demonstrated if the amount of pulling force necessary to separate the metal from the substrate is greater than 500 lb/in2. Consequently, a process for fabricating printed circuits in which metal is deposited on a substrate such that the amount of pulling force needed to separate the metal from the substrate is at least about 500 lb/in2 is desired.
A process for electrolessly forming metal on a substrates is disclosed. The amount of pulling force required to separate the substrate from the metal formed on the substrate by the present process is at least about 500 lb/in2. Typically, the metal is electrolessly formed on a substrate that is either made of a ceramic material or a substrate over which is formed a layer of resistive material. The surface on which the metal is electrolessly formed is referred to generally herein as a substrate. If the substrate is made of a ceramic material, it is advantageous if the substrate is cleaned before metal is electrolessly formed thereon. The substrate is cleaned either by heating it to a temperature of about 800xc2x0 C. to about 1500xc2x0 C. or by contacting it with a solution of aqueous base that is heated to a temperature of about 25xc2x0 C. to about 100xc2x0 C. If the substrate has a layer of resistive material formed thereon, the substrate is cleaned prior to the formation of the resistive material layer on the substrate.
The substrate surface is then treated with reagents that promote the electroless plating of metal on the substrate surface. Conventional techniques, for example treating the substrate surface with a sensitizing solution such as tin fluoride (SnF2) or tin chloride (SnCl2) followed by an activating solution of palladium chloride (PdCl2), are used for this purpose.
If the metal is to be formed in a pattern on the substrate, a layer of energy definable resist material is deposited on the substrate before it is treated with the sensitizing and activating solutions. A pattern is formed in the resist using conventional lithographic techniques. The pattern is developed, thereby exposing the portions of the substrate surface on which the metal is to be deposited in the desired pattern. When the substrate is then treated with the sensitizing and activating solutions, only the exposed portions of the substrate are contacted with these solutions. Consequently, the substrate surface is selectively sensitized in a pattern that corresponds to the resist pattern. The substrate with resist thereon is then dried using expedients such as baking the substrate at a temperature of at least about 100xc2x0 C. The resist is then stripped from the substrate. In an alternate embodiment a second resist plating mask is then formed over the selectively sensitized substrate surface in the manner described above. Specifically, the mask is formed at least over substantially all of the unsensitized portions of the substrate surface.
The substrate is then subjected to an electroless metal plating bath. It is advantageous if the electroless plating bath contains nickel although other electroless metal plating baths such as cobalt phosphorous are also contemplated as suitable. If the substrate has a second resist plating mask formed thereon, the metal that is deposited is laterally confined by the plating mask.
After the substrate has been selectively plated with metal, the substrate is then heated to a temperature of about 180xc2x0 C. to about 350xc2x0 C. for an amount of time that is sufficient for the metal to adhere to the ceramic substrate. The amount of time depends upon the temperature, i.e. the higher the temperature, the shorter the time. For the temperature range specified, the amount of time varies from about 30 minutes to about 24 hours.
It is advantageous if a second layer of metal is deposited on the first metal (e.g. nickel) layer previously plated on the substrate. Typically these metals are deposited by electroless plating. Copper and palladium are examples of these metals. It is advantageous if nickel is electrolessly plated over the copper metal to protect it from oxidation.
The substrate is then baked at a temperature of about 180xc2x0 C. to about 350xc2x0 C. The substrate is then baked again at a temperature and for a time in the ranges specified above.
In an alternate embodiment, the fire-cleaned, ceramic substrate is first treated with the activating and sensitizing solutions as described above. A layer of nickel is then electrolessly plated thereon and the substrate is baked at a temperature of about 180xc2x0 C. to about 350xc2x0 C. A patterned layer of resist material is then formed on the metal layer using conventional materials and techniques. The patterned layer is transferred into the underlying metal layer using conventional expedients such as reactive ion etching and chemical etching. The remaining portion of the resist material is then removed from the substrate using conventional techniques. Optionally, a layer of copper is electrolessly formed on the patterned nickel layer as described above. The substrate is again baked at a temperature of about 180xc2x0 C. to a temperature of about 350xc2x0 C. after the copper layer is formed thereon.